Assuming your car is a 2006 model none of the other Ford Diesel engines will be plug and play. Installing a different type of Ford Diesel engine requires a lot of parts and labour.
To install the much better 1.8 TDCI for example you need at least the following parts:
* 1.8 TDCI Engine including all parts, wiring harness and auxiliaries.
* 1.8 TDCI PCM.
* Engine bay wiring harness which is suitable for the 1.8 TDCI.
* Gearbox which has the correct gear ratios for the 1.8 TDCI.
* Complete exhaust system from front to back.
* Complete air intake system including air filter box, hoses and intercooler.
* Fuel lines.
Next to this the 1.8 TDCI has different springs and larger brake discs than the 1.6 TDCI. These parts should also be changed.
In my opinion you should only perform this conversion if you have access to a suitable donor car and loads of time. Otherwise it will be way too expensive.

Some time ago I installed a 12 Volt socket in the boot of my 2013 Focus MK3 hatchback.
I connected the 12 Volt socket to the fuse box in the boot. This fuse box should have at least a few empty positions which can be used to connect additional equipment.
My 12 Volt socket is connected exacly the same way as the 12 Volt socket of the Focus MK3 wagon (which has a 12 Volt socket as standard). The 12 Volt socket in the boot has a permanent 12 Volt power supply. All other 12 Voilt sockets in the car have a switched 12 Volt power supply (only 12 Volt if the CANbus system is active).

The amount of work involved in retrofitting the Premium (Sony) audio system highly depends on the audio system that is currently installed into the car.
The Premium (Sony) audio system is only compatible with the High Level audio system (with or without Sat Nav). The High Level audio system of the Focus MK3 has a 10 digit keypad and is always combined with either Bluetooth/Voicecontrol or SYNC 1.0/1.1. The High Level audio system of the Focus MK3.5 has a 8 Inch touchscreen in combination with SYNC 2.0.
The Sony audio system is not compatible with the Low Level audio system. The Low Level audio system of both the MK3 and MK3.5 has a 6 digit keypad and no bluetooth functionality. The Low Level audio system has different/less wiring than the High Level and Sony audio systems.

Most likely a defective GPS antenna. The GPS antenna is fully integrated into the antenna base on top of the roof.
A defective GPS antenna is a known problem of most Ford models. During time the antenna base suffers from deteriorating seals which causes water ingress into the antenna base. As a result of the water ingress the GPS antenna circuit board corrodes and the GPS antenna will no longer work correctly.
Because of this known problem Ford no longer produces the original Focus MK2/MK2.5 GPS antenna base. As a solution for this problem ford introduced a GPS antenna replacement kit which basically consists of a normal FM antenna base and a seperate GPS antenna which must be installed behind the windscreen. Unfortunately the Ford GPS antenna replacement kit is quite expensive.
As a much cheaper alternative you can install an aftermarket GPS antenna with a Blue Fakra connector. These aftermarket GPS antennas can be found on Ebay for under €10,- including P&P. An aftermarket GPS antenna can easily be installed behind the A-pillar cover. This way the GPS antenna is invisible and the GPS reception is still great. Most times even better than the GPS reception of the original GPS antenna.

The fact that you mention a button on the instrument cluster suggests that your car has the Level 1 instrument cluster. The Level 1 instrument cluster does not have a board computer so it is completely normal that you can not find it.
The Focus MK2 was available with 3 different types of instrument cluster. Which type of instrument cluster is installed depends on trim level and selected options of the car.
Level 1 instrument cluster. This type of instrument cluster has a small LCD display. This type of instrument cluster is installed to the lower trim levels and does not have a board computer. This type of instrument cluster has a button to reset the trip mileage counter.
Level 2 instrument cluster. This type of instrument cluster has a larger LCD display. This type of instrument cluster is installed to the higher trim levels and has a board computer which is controlled by buttons on the indicator stalk.
Level 3 instrument cluster. This type of instrument cluster has a dual LCD display. This type of instrument cluster is installed to the higher trim levels with factory satnav and/or automatic transmission. This instrument cluster has the same functionality as the Level 2 instrument cluster. The 2nd display is used to display satnav and/or automatic transmission info and also shows date/time.

The early Focus MK2 versions did have flat panel type air filters. Every type of engine basically has a slightly different type of air filter. After the interior facelift (early 2007) all engine types (except the Focus ST) have the same round type air filter. This round type air filter is currently still used on all Focus MK3/MK3.5 versions.

The MK2/MK2.5 radio's have completely different dimensions and wiring than the MK1/MK1.5 radio's. This requires major modifications to the dashboard and dashboard panel and wiring.
Next to this the MK2/MK2.5 radio's are technically completely different from the MK1/MK1.5 radio's. The MK2/MK2.5 radio's are connected to the CANbus system. Several data is communicated to and from the radio by using the CANbus system. The MK1/MK1.5 does not have a CANbus system at all so many functions will simply not work. The time and date function for example is integrated in both the radio and the instrument cluster. The MK2/MK2.5 radio shows the time and date that is stored in the instrument cluster memory. Without a MK2/MK2.5 instrument cluster the date/time function will not work correctly.
The steering wheel control however will work fine. The steering wheel control is basically a resistance circuitry. Every steering wheel control function has its own resistance. The resistances of each function are basically the same for most Ford models.

A suitable VIN number can easily be obtained from the Ford Etis website. You can enter the registration number of a Focus MK3 that has the SYNC 1.1 system on the Ford Etis website. The Etis website shows all specifications of that particular car including the VIN number.

Be aware that updates are only available for the SYNC 1.1 system onwards. The older SYNC 1.0 system can not be updated by the user. Updates for the SYNC 1.0 system can only be performed by a Ford dealer. If your car originally had the SYNC 1.0 system the Ford India and Australia websites will identifiy the car as a SYNC 1.0 vehicle based on the VIN number. A simple workarround for this is to make another user account using a VIN number of a Focus MK3 that has the SYNC 1.1 system.
The latest availabel update for the SYNC 1.1 system is version v5.9 (file name 8L2T-14D546-AF) . This update can be downloaded from the following link:
https://www.dropbox.com/s/71ntry1hxz2yeub/8L2T-14D546-AF.zip?dl=0

I registered on both the Ford India and Ford USA websites using my European VIN number without any problems. The latest USA SYNC 1.1 update is no longer suitable for European vehicles and the Ford India website is completely changed. The SYNC updates are no longer present on my user account.
Ford recently added SYNC support on the Ford Australia website. On this website it is still possible to make a user account and download the SYNC update files.
If needed I can also search for the update files on my computer and upload them .

DTC code U2100 is quite common. This DTC code is present in many FCDIM modules. Even on factory fitted FCDIM modules this DTC code is often present. This DTC code should not affect any functionality of the FCDIM module. Until now I have not found the cause of this DTC code or a possible solution. Personally I would just ignore this DTC code.
On a SYNC 1.1 (APIM) module the APPlink function should work. However not all mobile devices are fully supported. If not already done I would update the SYNC 1.1 module to the latest software version first. The latest software solves a lot of issues. For example on older software versions Apple products can only be connected to APPlink using USB while Android products can be connected by Bluetooth. On the later software versions both Apple and Android can connect by Bluetooth.

ELMconfig can be downloaded from the following website:
http://www.civil.iffc.ru/elmconfig/
This is the website of the author of the ELMconfig program. This website can not be viewed correctly using the Microsoft Edge browser so you need to use another browser. To make the ELMconfig program work you may need to install the runtimepack first.

When the gearbox is removed to replace the clutch and DMF assembly it is good practice to clean the bellhousing. Cleaning the bellhousing will only take 10 minutes maximum when the gearbox is removed. Personally I would be glad to pay for the extra 10 minutes labour to clean the bellhousing.
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A front parking camera was never available on the Focus MK3/MK3.5. Only a rear parking camera was available as an option (or as part of an option pack).
You car either does not have a front parking camera or if it has a front parking camera it must be aftermarket and not original.